As a soft magnetic material for cores of motors and transformers, a magnetic steel sheet is generally used at low drive frequencies of several kilohertz or less. In addition, at high frequencies of several tens of kilohertz or more, an oxide magnetic material, such as Mn—Zn-based ferrite, is generally used.
On the other hand, dust cores formed by compaction of iron powders are used at several tens of kilohertz or less in many cases. Since being formable by die-molding, the dust core has a very high degree of freedom of a product shape, and since even a complicated core shape can be manufactured by a simple process with high precision, the usefulness of the dust core has drawn attention.
One of important factors determining the properties of the dust core as described above is the iron loss, and in order to realize high performance (that is, reduction of iron loss) of the dust core, various proposals have been made on iron powders.
For example, in Japanese Unexamined Patent Application Publication No. 2003-217919 (Patent Document 1), a technique for reducing the iron loss has been disclosed in which Si is contained in iron powder particles and an insulating material primarily composed of SiO2 and MgO is provided between the iron powder particles. In addition, in Japanese Unexamined Patent Application Publication No. 11-87123 (Patent Document 2), a technique for improving the initial permeability (having an influence on the iron loss) in a high frequency region has been disclosed in which the content of Si and the distribution thereof are controlled so that the Si concentration at the surface portion is higher than that at the central portion.
When the dust core is manufactured, iron powder particles are preferably insulated from each other, and as an insulating method, for example, there may be mentioned a method in which after an insulating material is mixed with the iron powder particles, compaction is performed (for example, see the above Patent Document 1). In addition, as another insulating method, an iron powder for compacted iron powder processed by insulation coating has also been proposed. For example, in Japanese Unexamined Patent Application Publication No. 2003-303711 (Patent Document 3), an iron-base powder covered with a coating film containing a silicone resin and pigment has been proposed.
In addition, in Japanese Unexamined Patent Application Publication No. 2007-231330 (Patent Document 4), as a method for manufacturing a metal powder for dust cores, a technique has been disclosed in which Si is enriched on the surface of the metal powder by a gas-phase reaction and in which an insulation coating treatment is further performed whenever necessary. In the Patent Document 4, it has also been disclosed that when the surfaces of the powder particles processed by the gas-phase reaction are oxidized to form SiO2, heat generation of fine particles can be avoided, and/or the adhesion to an insulation coating material can be improved. However, examples in which the effects described above are verified have not been disclosed.